LieutenantSulu picks up a ship on sensors. The small craft of Denevan configuration is on course directly for the Denevan sun, and does not appear to be out of control. Kirk orders Sulu engage the Enterprise at warp 8 interception course. The Denevan vessel is out of range of the tractor beam; the Enterprise pursues. Finally, they make contact: seconds before the ship burns up, the pilot cries out "I did it! It's finally gone! I'm free!!" After the vessel's destruction, DoctorMcCoy walks up to Kirk and softly asks, "Jim, your brother Sam and his family – aren't they stationed on this planet?"

Spock and Montgomery Scott inform Kirk about Deneva's history and the fact that there has not been contact in a year. Uhura reports activity on the private transmitter channel, a woman speaking but does not respond to Kirk. Contact is abruptly cut; Kirk demands that Uhura get the channel open again, but she reports that it was shut off on the other end: the only thing she can do is keep hailing and hope they choose to answer back. Kirk identifies the woman as likely being his brother's wife Aurelan Kirk, to McCoy.

Once the Enterprise arrives at Deneva, Kirk forms a landing party consisting of him, Spock, McCoy, Scott, YeomanZahra, and Bobby. Spock says there are the expected number of Human life signs, but "strangely quiet." They beam directly into the capital city on Deneva. Once on the planet, they are struck by the curious lack of people; in a city of 100,000, no one is visible – until, a few minutes later, they are about to be attacked by four men who, even as they charge, scream "Go away! We don't want to hurt you!" But, with their clubs, they start to try anyway, forcing the landing party to stun them with their phasers, an attitude inconsistent with their actions. Then McCoy discovers with his medical tricorder that the nervous systems of the unconscious men are violently active – as if they are somehow still being stimulated.

A loud scream draws them next to Kirk's brother's lab. Kirk's brother, Sam, lies dead on the floor. Aurelan is hysterical, and their young son Peter is unconscious nearby. Kirk holds Aurelan close while McCoy sedates her. McCoy and Spock both give their condolences to Kirk. Evidence suggests something has been trying to force its way in through a ventilation shaft, despite the fact that the sensors showed nothing on Deneva that didn't belong there. McCoy recommends that Kirk returns to the ship with his relatives and the doctor so he can be present when Aurelan wakes. Spock is left in charge of the landing party.

In sickbay aboard the Enterprise, McCoy is waiting on lab results to identify the cause. In the meantime, the Kirks have been heavily sedated for the extreme pain they're experiencing, but Aurelan is surprisingly resistant to the tranquilizer and is still awake. Holding her hand, Kirk informs her that Sam is dead, but Peter is still alive. In terrible pain, she tells Kirk that "things" came, eight months prior, on a ship from Ingraham B as its crew had been forced to travel to Deneva. As she tries to answer Kirk's questions, she experiences more and more pain, until McCoy is forced to sedate her, which only blunts the pain. The creatures use the Denevans as their arms and legs, and are forcing them to build ships. They control their hosts with pain. Aurelan's last act is to implore Kirk not to let the things go any further; this effort costs her everything she has left, and with a pained scream she dies.

Kirk rejoins the landing party; he knows there is some sort of creature present, but the landing party has not yet discovered anything beyond a curious buzzing. Kirk orders their phasers to be set to force 3, on the kill setting as the creatures responsible have already shown a willingness to kill. Entering a building where they heard this sound, the landing party discovers strange creatures clinging to walls and the underside of an opening. Looking like little more than loathsome blobs of jelly, they emit an unwholesome buzzing, and employ a crude, wingless flight. Each armed party member fires at the creatures, but force 3 – sufficient to destroy most organisms – barely affects them, even after several seconds of exposure; one simply falls to the ground. And the creatures do not even register on Spock's tricorder.

"Spock, are you all right?!"

Thinking it may be a trap, Kirk orders the landing party out of the infested area; as they leave, the creature on the ground flies up and strikes Spock in the back, leaving a strange puncture wound. Kirk rips it off of Spock's back and asks the Vulcan if he is all right. Spock looks upward in pain.

In sickbay, McCoy removes a small strand of tissue from Spock's back, and then, over NurseChapel's objections, he closes the wound. The creatures attack by stinging; they leave behind a piece of this tissue that rapidly infiltrates the victim's entire nervous system, far too completely for conventional surgery to remove. McCoy explains this to Kirk on the bridge while showing him the piece of tissue and an identical piece removed from Aurelan's body in a container. He admits that he, the lab and the science department are all stumped on what to do next.

Spock recovers consciousness, rages past Chapel out of sickbay and storms the bridge. His goal: to take the ship out of orbit, throwing aside Sulu from the helm. Forewarned by Chapel, Sulu, Kirk, and two other officers grapple with Spock but his Vulcan strength proves troublesome and he nearly takes out Kirk with the Vulcan nerve pinch. With difficulty he is ultimately wrestled to the deck; Chapel arrives on the bridge with tranquilizer and McCoy sedates Spock as he's being held down. Kirk orders that he be returned to sickbay and security restraints employed, where McCoy makes another grim discovery. The K3 indicator, a measure of pain, is very, very high. The reason for the madness is confirmed: victims are in such agony that their minds eventually break under the stress. Spock, recovering consciousness, apologizes for his loss of control and now claims the ability to control the pain, noting that for Vulcans, "pain is a thing of the mind: the mind can be controlled". Referring to the creatures as the creature, only of thousands of parts, wants control of the ship which Spock is resisting. But after his visit to the bridge, and McCoy unsure if he could maintain control as time wears on, Kirk isn't sure and for the time being, has him continued to be confined to sickbay under observation. Kirk is intent on a cure for his nephew, but McCoy reminds him that the rest of the colonists deserve just as much consideration.

After they both leave, Spock, conquering the pain, breaks out of sickbay and getting back in uniform, plans to visit the planet's surface with a specimen case. Scotty, acting on Kirk's orders, refuses to transport him. A scuffle breaks out, including Spock nerve pinching Leslie. Scott holds Spock by threatening to stun him with a phaser and calls for Kirk to come down. Later, when Kirk appears, Spock explains that his plan is to retrieve a creature for study. He believes that since his nervous system is already infiltrated, there is little more the creatures can do to him. Kirk is convinced, and over McCoy's objections, Spock beams down to collect a creature for study, armed with Scotty's phaser.

Spock is attacked by a colonist on Deneva almost immediately after being beamed down. The man knocks Spock's phaser out of his hand with his club. Spock is able to incapacitate the crazed man with a nerve pinch and, upon regaining his phaser, begins to struggle against fresh pain from the parasite, having failed to stop Spock through the colonist. Spock knocks down a creature with his phaser fire, returns to the ship and begins to study it. Immediately, in one of the Enterprise's science labs, he realizes that the creature resembles, more than anything, an enormous brain cell. Kirk catches on immediately: these creatures are not separate animals, they are all parts of a single entity, connected together in some mysterious fashion. This is how it resists phaser fire: each part draws strength from the whole. Remembering how the colonist that flew into the sun cried out that he was 'free', Kirk feels that this is an angle they can work with. He orders an analysis of everything from the medical and life science departments in an hour.

McCoy's multiple efforts to find some method to kill the creatures fail. Not heat, not radiation – nothing can kill it. Kirk knows that if they cannot find a way to kill these creatures, he will be forced to destroy Deneva to prevent their spread. A million people will die if nothing can be done. Kirk cannot let the creatures spread and has absolutely no wish to kill the Denevans, including his nephew. He demands a third alternative.

Spock and McCoy report to Kirk's quarters, having not gotten any breakthroughs. Knowing the kind of pain Peter will experience if he wakes up, and admitting he's not sure how much longer he can hold out against the pain, Spock requests beaming down to Deneva and taking Peter with him before the planet is destroyed, but Kirk flatly denies permission. Kirk posits that the key must lie in exploring the properties of the sun. The Denevan the Enterprise contacted earlier was free of the creature moments before he died; something in the sun killed it. It is not radiation, it is not heat – could it be light? After turning a computer, with its flashing light, in his quarters on and off, Kirk thinks that it may be the answer. McCoy protests that light isn't lethal, but Kirk points outs it's just not to Humans; the creatures hide indoors and in shadows on Deneva. What if it's because they're sensitive to light, and strong amounts of light are lethal to them? Spock agrees it's a possibility. McCoy points out that they can't bring the planet to the sun, but Spock agrees with Kirk that they can bring the equivalent to Deneva, through a string of satellites around the planet, burning tri-magnesite and trevium.

McCoy rigs a test cubicle in the biolab, and Chapel puts the sample creature inside. Spock has calculated that at the distance that the Denevan declared himself free, the sun's brightness would be measured at 1,000,000 candles per square inch, and with the satellite network set to that intensity, anything within a closed, darkened area will be affected. With everyone putting on safety goggles, they expose the creature for several seconds and confirm the theory: high-intensity light is fatal to the creatures. As it's necessary to see what will happen to tissue that has infiltrated a victim, Spock volunteers to enter the cubicle – McCoy, who would prefer his guinea pig be someone other than the man he regards as Starfleet's best first officer, attempts to dissuade him, but Spock insists. He also refuses goggles, as none of the colonists will have any (and it would likely be time prohibitive to outfit a million people anyway), despite knowing that the intensity will damage his optic nerves. This test does, in fact, succeed: the blinding light completely frees Spock of the creature and the pain – but at the cost of his eyesight. Spock, exiting the cubicle, assures McCoy that it was an equitable trade – the closest he comes to revealing how much pain he has been experiencing. Only then is the true tragedy revealed: the lab test results on the dead creature delivered by Chapel indicate that the creatures are vulnerable only to a specific subset of the light spectrum: ultraviolet light is its Achilles' heel. McCoy is chagrined to realize that Spock need not have been blinded at all, but Spock reminds him that it was also his choice; what's done is done.

Despite this, the answer is at hand. Kirk orders satellite control to deploy a formation of 210 ultraviolet satellites at 72 miles altitude, in a permanent orbit. The satellites are turned on; the creatures begin to fail, to fall, to smoke and to die. Ground stations on Deneva quickly make contact; the creatures are dying everywhere. Kirk passes the good news down to sickbay and for McCoy to pass it on to Spock; he reminds McCoy that Spock's blindness wasn't his fault, but McCoy doesn't respond.

A little while later as Kirk has Zahra begin recording a mission update for Starfleet Command, she notices Spock returning to the bridge – he can once again see. It seems that an inner eyelid, a hereditary trait of Vulcans developed as a protection against the brightness of Vulcan's sun, closed to shield his eyes on instinct. It's normally thought about as much as a Human would think about their own appendix: completely ignored. Kirk ponders that regaining one's sight would be an emotional experience for most, and assumes that Spock, as usual, had none, but Spock corrects him: he had a very strong reaction as the first thing he saw was McCoy's face as the doctor was bending over him. McCoy replies that it's a pity that Spock's brief blindness didn't increase his appreciation for beauty. Interrupting the two, Kirk has Spock lay in a course for Starbase 10. Relieved beyond words, McCoy asks Kirk not to mention his previous "best first officer" statement – at which point Spock turns around from his science station and thanks the doctor for his compliment, and Kirk jokes that all the concern over Spock's eyes had led McCoy to forget about his Vulcan ears. Kirk has Sulu take the Enterprise to warp factor 1 and the ship leaves Deneva.

"Captain's log, stardate 3287.2. The mass insanity we have tracked across this section of the galaxy seems to have already touched Deneva. That planet, colonized over a century ago, is one of the most beautiful in the galaxy."

"Captain's log, supplemental. Whatever the creatures are, they have apparently taken over all the inhabitants of Deneva. Meanwhile, ship's surgeon Dr. McCoy is examining a strange puncture wound left by one of the creatures on Mr. Spock's back."

"Captain's log, stardate 3289.8. I am faced with one of the most difficult decision of my life, unless we find a way to destroy the creatures without killing their Human hosts, my command responsibilities will force me to kill over a million people."

This is the only episode title of the Original Series that includes an exclamation point and, along with VOY: "Bride of Chaotica!", is one of only two episodes in the entire Star Trek franchise to incorporate that particular punctuation mark. It is also the only episode of the original series to incorporate a dash in the title.

In addition, it is one of only three Original Series episodes in which the word "damn" is used, with McCoy cursing the "damnable logic" behind the experiments designed to destroy the creatures. The only other uses of the word "damn" occur in "Journey to Babel", wherein Kirk admits he can't "damn" Spock for his loyalty by taking command of the Enterprise at the cost of Sarek's life, and in "Court Martial" where Kirk makes a log entry stating that the evidence against him is "damning."

Deleted scene with Peter Kirk in captain's chair

A filmed scene cut from this episode featured dialogue between Kirk and his nephew Peter. The dialogue concerned Peter's returning to Deneva to live with Sam Kirk's partner. [1]

Steven W. Carabatsos had an obligation in his contract, that he must deliver at least one script of his own while serving as story editor. Carabatsos left the series in late-1966, but still had to fulfill this task before departing. Having no idea of his own, Gene Roddenberry suggested him one, entitled Operation: Destroy!, which was the basis for this episode. (These Are the Voyages: TOS Season One)

The script, as originally conceived, did not envision the parasites being killed with light. Instead, the Enterprise leaves Deneva and destroys their home planet. By effectively annihilating the central "brain" that controls their operations, the ship renders the creatures harmless. Working from an early draft of the script, James Blish writes up this version in his adaptation of the episode in his volume Star Trek 2.

The first draft script, entitled "Operation: Destroy", did not feature Kirk's brother and his family. In that version, a Denevan woman named Aurelan was in love with a young man named Kartan, who flew his ship into the Sun to destroy the creatures. Aurelan and her father were not infected, and helped the Enterprise crew in their research. (The Star Trek Compendium, p. 65)

The Deneva outdoor scenes were shot at the headquarters of TRW Space and Defense Park in Redondo Beach, California (currently the Northrop Grumman Space Technology headquarters). See here for the location in Google Earth. The establishing shot of Kirk's brother's lab was a building on the campus of UCLA, and the entrance of the building was the cafeteria at TRW. (Star Trek Encyclopedia (2nd ed., p. 112)) The scenes at TRW were filmed 15 February1967. [2]

This is the first time McCoy's lab is seen. Inside the lab, the prop used previously as Balok's lamp device in "The Corbomite Maneuver" can be seen sitting on a shelf. Different components of sickbay were added over the first season, such as the decompression chamber seen in "Space Seed". McCoy's lab contains one of the life support canisters used on the Botany Bay.

The chairs that were seen in Chief Vanderberg's office in "The Devil in the Dark" were recycled for Sam Kirk's office, albeit painted pink.

The oversized plexiglas tongs that Spock uses to pick up the Denevan parasite would again be used to recover the phasered sample of tritanium in the teaser of "Obsession".

The starmap showing the progression of the space madness also shows up on station viewscreens in future episodes.

Unique to this episode, Uhura wears a black belt on the bridge, having worn one while on the landing party in the previous episode produced, "The City on the Edge of Forever". In "The Gamesters of Triskelion", she dons it when she joins the landing party and wears a communicator and phaser.

Although the ubiquitous jumpsuit-type outfits worn by the four Denevans who attack the landing party crop up in a variety of colors, this is one of the few in which a green one appears. There is also a red variety, apparently the only one ever seen in The Original Series.

There is a blooper from this episode in the first season blooper reel. The parasite creature that was supposed to hit Leonard Nimoy's back is seen hitting his backside instead, making Nimoy break up with laughter. Also appearing is a sequence showing the landing party with their phasers being used in tandem as electric shavers.

McCoy speaks about "getting the plates back" on Kirk's nephew to help with his medical treatment. This line references X-ray plates which were common in the 1960s but, in the 2260s, would have been extremely archaic and impractical compared to other medical advances of the time (such as DNA examination).

The Vulcan inner eyelid is mentioned again in ENT: "The Forge". In the Star Trek novel Spock's World, the inner eyelid was developed in one tribe of Vulcans who eventually took control over most of the planet.